1. Field of the Invention
The present invention relates to a semiconductor module used to control a high electric current, and more particularly to a non-insulating semiconductor module to which a protection circuit for protecting an internal circuit can be attached.
2. Description of the Related Art
A semiconductor module is widely used not only to process control signals but also to control high currents. For example, in a controlling device which drives a motor for running an electromotive car such as a battery-powered forklift, etc., a semiconductor module with a large electric capacity is used as a switching device arranged between the battery and the motor at the last stage of a controlling circuit.
As a semiconductor module of this type, a variety of configurations are known: a configuration where a large electric current capacity is obtained by connecting a lot of identical power semiconductor chips in parallel; a configuration where simple circuitry is formed by using several types of a semiconductor chip; a configuration where a driving circuit of a semiconductor chip is included, etc.
A semiconductor module, especially a power semiconductor module is normally formed by putting the above described semiconductor chips into a resinous package. A package is normally made of plastic, and its internal semiconductor chip is insulated by ceramic, etc. The vacant space within the package is filled with gel or epoxy resin, etc. in order to prevent the semiconductor chip and its peripheral circuits from being oxidized.
Since a high current flows into the power semiconductor module, the amount of generated heat becomes large. Accordingly, its heat radiation must be considered. As one configuration for radiating the heat of a semiconductor module, a method for mounting a semiconductor module on a base substrate with a large thermal capacity and a high radiation effect (that is, with high thermal conductivity) is frequently adopted. In this case, the heat generated by the semiconductor module is radiated via the base substrate.
FIG. 1 exemplifies a perspective view of the internal structure of a semiconductor module. Here, a MOSFET is shown as an example.
A semiconductor module 1 comprises a plurality of semiconductor chips 2. The plurality of semiconductor chips 2 are mounted on the upper surface of a base substrate 3. The bottom of each of the semiconductor chips 2 is a drain area, and the base substrate 3 is a conductor (metal plate). Accordingly, the base substrate 3 is used as a drain electrode of the semiconductor module 1. The semiconductor module having this configuration is sometimes referred to as a non-insulating semiconductor module.
A source electrode 4 and a gate electrode 5 are arranged on the upper surface of the base substrate 3. Insulating plates 7 are respectively arranged between the source electrode 4 and the base substrate 3 and between the gate electrode 5 and the base substrate 3. The source electrode 4 and the gate electrode 5 are respectively connected to the source and the gate areas of each of the semiconductor chips 2 by bonding wires 8.
The semiconductor module is included in the package as described above. The source electrode 4 and gate electrode 5 protrude from the top of the package to its outside so that they can be connected to external circuits, although this is not show in this figure. The drain electrode is the base substrate 3 as described above, and is located on the bottom of the semiconductor module. As stated above, the drain electrode of the non-insulating semiconductor module is normally located on the bottom of the package, while the other electrodes are located on the top of the package.
If a control signal is applied to the gate electrode 5 on the semiconductor module 1 having the above described configuration, each of the semiconductor chips 2 are turned on. As a result, a main current flows from the drain electrode (base substrate) 3 to the source electrode 4 via the bottoms of the semiconductor chips 2, the tops of the semiconductor chips 2, and the bonding wires 8.
When a semiconductor device is switched (especially, when a transistor is turned off), a surge voltage occurs. Additionally, since the inductor (inductance) caused by the bonding wires, etc. exists in the semiconductor module 1, a considerably large surge voltage may sometimes occur when the semiconductor chips 2 are turned off. Such a surge voltage damages the semiconductor chips electrically and thermally.
To protect the semiconductor chips 2 from the above described surge voltage, a protection circuit such as a snubber circuit, etc. is frequently arranged. The snubber circuit has, for example, the configuration shown in FIG. 2, and absorbs a surge voltage.
FIG. 3 shows the configuration of the semiconductor module and its protection circuit. A protection circuit 11 is arranged at the side of the semiconductor module 1. Additionally, the protection circuit 11 is, for example, the snubber circuit shown in FIG. 2. In this case, the protection circuit 11 must be connected to the source and the drain electrodes of the semiconductor module 1. In the example shown in FIG. 3, the protection circuit 11 and the source electrode 4 are connected by a wire 12, while the protection circuit 11 and the base substrate 3 being the drain electrode are connected by a wire 13. It may be configured that the base substrate 3 is connected to a good conductor 14 such as an aluminum block, etc., and the protection circuit 11 and its good conductor 14 are connected instead of directly connecting the protection circuit 11 and the base substrate 3.
However, the source electrode 4 of the semiconductor module 1 having the above described configuration is arranged above the package, and the base substrate 3 being the drain electrode is arranged below the package. Therefore, the lengths of the wires 12 and 13 cannot be shorter than a predetermined length. As is well known, as the lengths of the wires 12 and 13 become longer, their inductances become larger. When the inductances increase, the capability of the protection circuit 11 for absorbing a surge voltage deteriorates.
As described above, in the conventional configuration, the capability of the protection circuit 11 cannot be fully exhibited, so that a surge voltage is not sufficiently absorbed in some cases. An attempt to improve the capability of the protection circuit 11 causes the size or the cost of the circuit itself to increase.
Furthermore, if the protection circuit 11 is arranged at the side of the semiconductor module 1, the area for arranging the protection circuit 11 naturally becomes wider, which hinders a device size reduction.